1. Field of the Invention
The present invention relates to a radiation imaging apparatus and imaging method using the radiation, and in particular, to the radiation imaging apparatus and imaging method using the radiation in which and by which radiation data is obtained by radiation-scanning an object in plural directions, the obtained radiation data are processed with a tomosynthesis method to reconstruct topographic data, the reconstructed tomographic data being used to identify three-dimensional positions of internal structures of the object.
2. Background Art
In recent years, tomographic imaging using a tomosynthesis technique has been used actively. The theory of this tomosynthesis technique has been known long before (for example, refer to patent reference 1), and recently, tomographic imaging that enjoys ease of image reconstruction performed using the tomosynthesis technique has been proposed (for example, refer to patent references 2 and 3). Especially, many such cases can be found in dental and mammographic fields (for example, refer to patent references 4, 5 and 6).
In the dental field, tomosynthesis technique is usually put into practical use as a panoramic imaging apparatus that acquires panoramic images in which a curved tooth row is usually expanded into a two-dimensional plane. This panoramic imaging apparatus is usually provided with a mechanism that rotates a pair of an X-ray tube and an X-ray detector around the oral cavity of an object being imaged. The X-ray detector has pixels mapped in a rectangle of a portrait-oriented width. The mechanism rotates the pair of the X-ray tube and the X-ray detector with a rotation center thereof intricately so that the rotation center traces a predetermined orbit which is previously set along a tooth row. The predetermined orbit is set to focus on a 3D referential tomographic plane previously set along a tooth row which can be regarded as a tooth row having a standard shape and size. During the rotation, an X-ray beam is radiated from the X-ray tube at given intervals and the X-ray is transmitted through the object to be received by the X-ray detector, and digital frame data is detected from the detector. In this way, the frame data focusing on the 3D referential tomographic plane is acquired at the given intervals. These frame data are subjected to reconstruction using the tomosynthesis technique so as to provide a panoramic image of the 3D referential tomographic plane.
However, the foregoing conventional panoramic imaging apparatuses do not take it account the facts that there are differences between the tooth row of each object and the 3D referential tomographic plane and positioning tooth rows involves difficult operations. As might be expected, there are individual differences in the shapes and sizes of the tooth rows of respective objects. The sizes of objects' jaws differ depending on the individuals, making it difficult to correctly position the tooth rows. This often causes defocused panoramic images to be reconstructed, which may fail to meet a demand for fine interpretation of the images. In such cases, if it is desired to finely examine cases including cavities and alveolar pyorrhea, it is needed to perform intraoral imaging or dental CT imaging, separately from the panoramic imaging. Re-performance of the panoramic imaging and X-ray imaging using another modality will raise the amount of X-rays to which the object is exposed.
In order to try to overcome such difficulties, there is provided an apparatus provided by patent reference 7. In the panoramic imaging apparatus shown in this publication, a phantom is used to previously measure gains (i.e., distance information for mutual addition of frame data) and positions in each of depth directions of a tooth row. Additionally, acquired frame data are used to produce a focus-optimized image of the 3D referential tomographic plane using the tomosynthesis technique. In the focus-optimized image, a ROI is set to specify a partial region, and a focus-optimized image at a selected position in the front-back direction (i.e., each of front-back directions of the tooth row, which connect the X-ray tube and the X-ray detector at each of the radiation positions) of the partial region are reconstructed using already acquired frame data and a gain necessary among the gains which have been measured. Hence, the data acquisition is performed one time with the 3D referential tomographic plane focused, and then, a focus-optimized image of any partial region can be reconstructed by making use of the already acquired frame data.